A role for the centrosome in regulating the rate of neuronal efferocytosis by microglia in vivo

Abstract

During brain development, many newborn neurons undergo apoptosis and are engulfed by microglia, the tissue-resident phagocytes of the brain, in a process known as efferocytosis. A hallmark of microglia is their highly branched morphology characterized by the presence of numerous dynamic extensions that these cells use for scanning the brain parenchyma and engulfing unwanted material. The mechanisms driving branch formation and apoptotic cell engulfment in microglia are unclear. By taking a live-imaging approach in zebrafish, we show that while microglia generate multiple microtubule-based branches, they only successfully engulf one apoptotic neuron at a time. Further investigation into the mechanism underlying this sequential engulfment revealed that targeted migration of the centrosome into one branch is predictive of phagosome formation and polarized vesicular trafficking. Moreover, experimentally doubling centrosomal numbers in microglia increases the rate of engulfment and even allows microglia to remove two neurons simultaneously, providing direct supporting evidence for a model where centrosomal migration is a rate-limiting step in branch-mediated efferocytosis. Conversely, light-mediated depolymerization of microtubules causes microglia to lose their typical branched morphology and switch to an alternative mode of engulfment, characterized by directed migration towards target neurons, revealing unexpected plasticity in their phagocytic ability. Finally, building on work focusing on the establishment of the immunological synapse, we identified a conserved signalling pathway underlying centrosomal movement in engulfing microglia.

In vivo imaging of the phagocytic dynamics underlying efficient clearance of adult‐born hippocampal granule cells by ramified microglia

“…However, the mechanism underlying this retrograde movement has not been clarified. In the zebrafish optic tectum, apoptotic neurons are removed by microglial phagocytosis, which is initiated by forming phagocytic cups without translocation of the cell body, similar to the phagocytic behavior of ramified microglia observed in the mouse SGZ (Möller et al, 2022). The experimental perturbation of the microtubule system disrupted the pattern of phagocytosis in zebrafish microglia.…”

Understanding the diversity and dynamics of in vivo efferocytosis: Insights from the fly embryo

“…Morphologically, they are highly spiky cells, closely resembling the microglia, which are the resident macrophages of the vertebrate CNS. 122,123 Furthermore, like the microglia, the Drosophila phagocytic glia appear to use their spiky, "filopodial" protrusions to reach out and grasp cellular debris (Figure 2). 78 While embryonic glial phagocytosis appears to be morphologically homogeneous, engulfment throughout embryogenesis can be far more diverse, as exhibited by the very dynamic and multifunctional macrophages.…”

“…They are embedded within a tightly packed tissue, where they are, unsurprisingly, immobile. Morphologically, they are highly spiky cells, closely resembling the microglia, which are the resident macrophages of the vertebrate CNS 122,123 . Furthermore, like the microglia, the Drosophila phagocytic glia appear to use their spiky, “filopodial” protrusions to reach out and grasp cellular debris (Figure 2).…”

Intercellular contact and cargo transfer between Müller glia and to microglia precede apoptotic cell clearance in the developing retina

“…Alternatively, Müller glia may actively sense and reach dying cells with cellular processes that then enclose around the target. In addition, given that we and others reported that microglia are the dominant phagocyte in the developing CNS (Blume et al, 2020; Casano et al, 2016; Mazaheri et al, 2014; Möller et al, 2022), there remains a strong possibility that interactions between microglia and other cell types such as the Müller glia occur during, and to facilitate, the clearance process. While early endosomal markers support active engulfment of dying cells by Müller glia in the absence of microglia (Thiel et al, 2022b), lysosomal fusion of such compartments upon engulfment within Müller glia has not been clearly demonstrated.…”